It has been proposed heretofore to line subterranean structures, generally referred to hereinafter as tunnels, with concrete in the course of manufacture or as a means of holding the previously formed structure.
One of the techniques used for this purpose is to pump concrete between a metal shell as it is advanced along the subterranean structure, the concrete filling the space between this shell and the rock or earth wall.
The concrete thus remains in place as a timbering while the tunnel is advanced in the desired direction, e.g. by tunnel excavating machines.
It is also known to leave a metal tunnel lining in place within the subterranean structure and to grout the space between this metal lining and the wall of the tunnel with concrete.
The present invention relates to improvement in such systems and, in general, to the provision of a reinforced concrete for the specific purpose of lining tunnel walls by pumped emplacement.
A tunnel wall which is lined with concrete is generally more stable than a wall built up or timbered from other structural elements and is highly desirable for the formation of water-carrying tunnels, tunnels for transport purposes or mine-shaft tunnels and the like.
The concrete structure within a tunnel must be capable of withstanding compressive forces which result from sinking of the earth, hydrostatic pressures and the like i.e. must be load bearing.
It has been the practice, as noted above, to pump the concrete between a shell and the subterranean structure wall, e.g. directly behind a tunnel-excavating machine. The machine shell may thus form the falsework around which the concrete lining is formed.
It has been proposed heretofore, in addition, to provide a structural-steel reinforcement in the space between the falsework and the subterranean structure, this reinforcing network being imbedded in the concrete.
However, in many instances, the provision of such reinforcing steel is not possible and, as a rule, tunnel linings are formed without steel reinforcements.
True, it is known to provide tunnel linings which are cast in place from concrete which contains steel fibers of a length of about 25 mm and a diameter of about 0.2 mm. This increases the tensile strength of the concrete but has not been found to significantly and positively affect the compressive strength thereof. In addition, the use of such fibers is disadvantageous since they seem to give rise to separating phenomena during the pumping operation.
It is recognized in concrete emplacement operations generally that separation of the components of the concrete for the "mix" is a significant problem.
In practice it has been observed with the use of such steel fibers and conventional tunnel-lining concretes that the steel fibers tend to become inhomogeneously distributed upon pumping of the concrete and tend to gather in porcupine-like balls. The regions of the hardened concrete at which such porcupine-like balls are located are found to constitute singular locations in the finished construction whose compressive strength is less than that of other locations and may be well below the minimum compressive strength required for the tunnel lining.
As a result of these singular locations, the tunnel lining does not have the desired degree of homogeneity or the isotropic characteristics which have been found to be necessary to resist the stresses to which the lining may be subject.
Attempts made to redistribute the steel fibers, e.g. by vibration or the like, have not proved to be completely successful from either an economic or a technological viewpoint.